Peavey PMA-200 amp loud buzz and DC on outputs

[flyingtele]

I have a Peavey PMA 200 and its not happy. When turned on, if the level controls are raised then the output meters light up to maximum, and if a speaker is connected then you get a very loud 100 Hz buzz that sounded like a squarewave. I measured 22 Volts across the terminals, at 100 Hz.
In trying to isolate the source of the loud buzz, I disconnected the cables leading into the power amp from the level control board to see if that was the source of the buzz. I still got 22V, but this time when I connected the speaker I got DC.
I reconnected the board with the led power indicators and gain controls, and once again got the loud buzz, but later on only got the DC output.
The amp has the Peavey DDT circuit, but I doubt that would have much to do with the problem.
The loud buzzing is constantly present at the output terminals regardless of the level controls, but when the level controls are turned up the level shown by the LED indicator will rise. I tested all of the output transistors and none are blown or shorted out.

I am not sure which direction I should approach this from, or where to start testing etc. I have searched the net for schematics or a service manual and found nothing. I have emailed Peavey requesting a copy of the service manual so hopefully then can send me that which would be helpful.

Any help, ideas, or other useful suggestions would be greatly appreciated. I am relatively new to electronics repair and only have basic knowledge, but always keen to learn so I would be most grateful for any help. Once I get the schematics I'll put them up.

[jackand08]

A loud buzz/hum at twice the mains frequency normally indicates a problem with the main power supply filter capacitors (old/leaky/high ESR) which means that the supply rails do not get filtered properly.

If you have an oscilloscope, it may help to scope the supply rails and check for anything other than DC that shouldn't be there. If you do find anything other than DC, then it more than likely will be the filter capacitors.

[flyingtele]

Thanks for your reply, but I don't think that's the problem. When I say there was a loud output buzz, there was 22 V across the speaker terminals. I can't see how old filter caps would cause that. After I disconnected the level control board, I got DC 22 V at the output terminals yet all the output transistors seem to be okay. Seems strange to me.

[Conrad Hoffman]

Go check your grounds. I just fixed a Peavey where the cable shield from the inputs on the rear panel connected to the main board with an IDC connector. The shield had a piece of tubing slipped over it, and then was inserted into the IDC connector. The connector couldn't properly cut through the tube and the ground connection eventually failed. This will give the symptoms you describe. Might not be your particular problem, but definitely check ground resistance between inputs and circuit board. Wiggle stuff (careful of voltages) and see if the problem changes.

[jackand08]

I can't see how old filter caps would cause that.

Old filter capacitors cause this by the electrolyte drying up, therefore decreasing their ability to pass currents to ground (and perform their job of filtering). This then manifests itself as a loud buzz coming from the speaker as the power supply to the output transistors is not clean. Depending on the age of the amplifier and the quality of the filter capacitors, they may be in need of replacement.

Also, what did you measure the 22V with? A DVM almost certainly display a varying AC hum as DC, whereas an oscilloscope will actually let you see the various components of the waveform.

As Conrad says too, a loud hum/buzz could also be caused by a really bad ground loop or dodgy ground connection.